1. Field of the Invention
The present invention relates to power inverter systems and methods of correcting supply voltage of such power inverter systems, which are capable of inverting an independent direct current (DC) voltage of each phase into multiphase (polyphase) alternative current (AC) power with variable voltage and frequency.
2. Description of the Related Art
Conventional multilevel inverter systems for reducing higher-order components and/or the threshold voltage of switching elements include single-phase inverter systems, inverter systems with three-phase star-connected single-phase multilevel inverters, and multilevel inverter systems with three-phase star-connected inverter units each of which has cascade-connected inverters.
FIG. 13 discloses a multilevel inverter with single-phase inverters 1U, 1V, and 1W that are three-phase star-connected to an AC motor 2. The inverter shown in FIG. 13 is operative to supply AC power with variable frequency and voltage from its output terminals U, V, and W to the AC motor 2.
An example of a control circuit for such multilevel inverters having a circuit structure as illustrated in FIG. 13 is disclosed in paragraph 6.2.4 of the New Drive Electronics. Tokyo: DENKISHOIN, 1984.
The control circuit is operative to separate a current to be supplied to an induction motor into a torque current component and a field current component (an exciting current component) and to independently control the torque current component and the field current component.
As illustrated in FIG. 13, when a single-phase inverter has a DC power source so that a DC voltage is independent for each phase, DC voltage ripples occur due to momentary power of each phase. These DC voltage ripples cause an output current from the single-phase inverter to become distorted from the original waveform (sinusoidal waveform).
The magnitude of ripples appearing in the DC voltage depend on a relationship between reactive power based on the output current from the single-phase inverter and reactances of reactive components, such as a smoothing capacitor and a power-source side reactor, of the single-phase inverter.
When the momentary power of a single-phase inverter is positive and the DC voltage is lower than a no-load voltage, the output power of the single-phase inverter should depend on the DC voltage supplied from the power source side thereof. However, if the reactance of the power-source side reactor is comparatively high, because the current to be supplied to the smoothing capacitor is delayed, the output power of the single-phase inverter depends on the energy charged in the smoothing capacitor. This causes the DC voltage to drop off severely.
In contrast, if the momentary power of the single-phase inverter is negative, a current based on the DC voltage supplied from the power source side flows through the smoothing capacitor so that the smoothing capacitor is charged. In this case, when the reactance of the power-source side reactor is comparatively high, even if the DC voltage is higher than the no-load voltage, the current keeps flowing through the smoothing capacitor with the DC voltage maintained higher than the no-load voltage.
That is, DC voltage ripples occur depending on the positive and negative variations of the momentary power of the single-phase inverter.
A frequency of the DC voltage ripples is proportional to a reactive power component of the output power. Specifically, the DC voltage ripples have a frequency, which is double the frequency of the fundamental of the output voltage or the output current.
If the frequency of the DC voltage ripples is sufficiently higher than a resonance frequency of the power side reactor and the smoothing capacitor, in other words, if the phase velocity of the DC voltage ripples is faster than the charge and discharge speed of each the power side reactor and the smoothing capacitor, the magnitude of the DC voltage ripples can be reduced.
In order to reduce the magnitude of the DC voltage ripples, comparatively high reactance of the power-source side reactor of each single-phase inverter and comparatively high capacitance of the smoothing capacitor thereof may be required, which can reduce the resonance frequency of the power-source side reactor and the smoothing capacitor of each single-phase inverter. Otherwise, it may be necessary to set the frequency of the no-load voltage of each single-phase inverter to be higher than the resonance frequency of the power-source side reactor and the smoothing capacitor of each single-phase inverter.
An increase of the reactance of the power-source side reactor of each single-phase inverter may however contribute to an increase of the DC voltage drop, deteriorating the power conversion efficiency of the power inverter system. Using a capacitor with a comparative high capacitance as the smoothing capacitor of each single-phase inverter may cause the circuit size of each single-phase inverter and the cost thereof to increase.